Increasing frequency of heavy precipitation events—especially where rain falls over large areas of impermeable surfaces—is one factor that increases the threat of urban flooding.
Picture of flooded homes in Libertyville, Illinois.

Homes on a floodplain in Libertyville, Illinois, suffered heavy damage in September, 2014.

In addition to the increased threat of flooding in coastal regions associated with sea level rise, climate-related changes in precipitation patterns have also increased the threat of flooding in some inland regions. The frequency and intensity of very heavy precipitation events have increased across most of the nation, and scientists project that these trends will continue. For instance, by late this century heavy precipitation events that historically occurred once in 20 years may occur as frequently as every 5 to 15 years. Consequently, the frequency of floods associated with heavy precipitation events is expected to increase. This includes urban floods, where relatively large areas of impermeable surfaces increase the volume of runoff, and flash floods that occur in relatively steep or small watersheds.

Roadway damaged by flooding on the Red River in Minnesota.

Flooding on the Red River in Moorhead, Minnesota, resulted in roadway damage.

The volume of runoff associated with increasingly heavy precipitation events has the potential to overwhelm decades-old infrastructure, such as culverts and bridges. Higher volumes of runoff can also overflow existing retention basins, challenging the capacity of stormwater systems. Some cities also face water quality issues related to stormwater runoff. This has prompted some communities to adopt strategies to keep more precipitation where it lands.

Excerpted and adapted from the report Climate Change Impacts in the United States: The Third National Climate Assessment (Chapter 2: Our Changing Climate and Chapter 3: Water Resources).

Banner Image Credit
Water flowing over a road between flooded cornfields near Mapleton, North Dakota in April, 2011. Marisa Lubeck, U.S. Geological Survey
Last modified
21 June 2017 - 3:16pm